Persistent organic pollutants (POPs) are chemicals that accumulate in the food chain and are toxic to humans and wildlife. The fourth World Health Organization (WHO) survey on POP levels in human milk (20062009) aims to provide baseline and trend information on human exposure to POPs. So far Belgium participated in all three previous rounds (1988, 1992, 2001). Whereas the first three rounds focused on determination of dioxins and PCBs in pooled (mixed) samples, the fourth survey comprised the analyses of individual milk samples for nine basic POPs (chlorinated pesticides and indicator PCBs) and of pooled milk samples for basic POPs, advanced POPs (dioxins and dioxin-like PCBs) and optional POPs (polybrominated diphenylethers [PBDEs], polybrominated dioxins and dibenzofurans [PBrDD/F], mixed halogenated dioxins and dibenzofurans [PXDD/F] and hexabromocyclododecane [HBCD]). For the Belgian participation human milk samples were collected during the summer of 2006 from 197 women between 18 and 30 years old distributed over all Belgian provinces. The individual samples were analyzed in a Belgian Laboratory for basic POPs. A pooled sample was made from 178 individual samples and analyzed by the WHO Reference Laboratory for the basic, advanced and optional POPs. The results indicate that most organochlorinated pesticides banned 2530 years ago were below or around detection limits in Belgian human milk samples although DDE was still found at low levels in all samples. Over the last five years the levels of marker PCBs and PCDD/Fs in Belgian human milk decreased, respectively, by 58% and 39%. For some of the other emerging or older compounds recent international data are needed to allow comparison. This shows the importance of international studies as run by WHO.